CHAPTER V 
THE STUDY OF BACTERIA AND GENERAL 
BACTERIOLOGIC TECHNIC 
I. Study of Bacteria in Living State 
The study of bacteria in living state is carried out in 
what is called the “hanging drop” preparation: A 
“hollow” slide is used—an ordinary glass slide, usually 
3 inches by 1 inch, in the center of which a circular de¬ 
pression is made; the edges of it are smeared with vase¬ 
line, then a drop of fluid medium in which the bacteria 
have been grown is placed in the center of an ordinary 
cover-glass (this is a piece of very thin glass, about 1 
inch square) ; the majority of books then suggest to lift 
up the cover-slip and then invert it over the concavity 
of the “hollow” slide, so that the drop hangs over the 
depression of the slide; my custom is to do just the oppo¬ 
site; namely, without disturbing the cover-glass, I lift up 
the slide and invert it over the cover-glass so that the 
vaseline around the depression of the slide just touches 
the cover-slip, and then put the slide down on the table 
with the cover-glass uppermost; in this way the danger 
of breaking the very thin cover-glass in handling it with 
fingers is eliminated (the danger of crushing it when 
inverting the glass slide over the cover-glass does not 
exist since the vaseline over the depression of the slide 
acts as a buffer between it and the cover-glass); besides, 
another source of danger is done away with; namely, 
that of spilling or smearing the bacterial drop while in- 
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